Nucleic Acid Biochemistry Practice Questions

Q: Which of the following is NOT present in DNA?

Uracil. ***Uracil*** - **Uracil** is a pyrimidine nitrogenous base that replaces **thymine** in RNA, meaning it is not found in the structure of DNA. - In RNA, uracil pairs with **adenine** through two hydrogen bonds. *Thymine* - **Thymine** is a pyrimidine found in DNA, where it pairs with **adenine**. - It is replaced by **uracil** in RNA. *Cytosine* - **Cytosine** is a pyrimidine that is a fundamental component of both **DNA** and **RNA**. - In DNA, cytosine pairs with **guanine**. *Adenine* - **Adenine** is a purine that is a fundamental component of both **DNA** and **RNA**. - In DNA, adenine pairs with **thymine**, and in RNA, it pairs with **uracil**.

Q: DNA and RNA both contain which pyrimidine base?

Cytosine. ***Cytosine*** - **Cytosine** is a **pyrimidine** base found in both **DNA** and **RNA**. - In DNA, cytosine pairs with **guanine**; in RNA, it also pairs with guanine. *Uracil* - **Uracil** is a **pyrimidine** base found only in **RNA**, replacing thymine. - It pairs with **adenine** in RNA. *Thymine* - **Thymine** is a **pyrimidine** base found only in **DNA**. - It pairs with **adenine** in DNA and is replaced by uracil in RNA. *Guanine* - **Guanine** is a **purine** base, not a pyrimidine. - It is present in both **DNA** and **RNA**, and pairs with cytosine.

Q: RNA with enzymatic activity is

Peptidyl transferase. ***Peptidyl transferase*** - The **peptidyl transferase** activity, resident in the **large ribosomal subunit (50S in prokaryotes, 60S in eukaryotes)**, is actually carried out by **ribosomal RNA (rRNA)**, making it a ribozyme. - This enzymatic activity catalyzes the formation of a **peptide bond** between the amino acid at the P-site and the incoming amino acyl-tRNA at the A-site during protein synthesis. *Peptidase* - **Peptidases** are enzymes that catalyze the **hydrolysis of peptide bonds**, breaking down proteins into smaller peptides or amino acids. - While essential for protein turnover and digestion, peptidases are typically **protein enzymes**, not RNA molecules. *Cytidine deaminase* - **Cytidine deaminase** is an enzyme that catalyzes the **deamination of cytidine** to uridine, a reaction crucial for pyrimidine metabolism and often targeted in cancer therapy. - This enzyme is a **protein**, not an RNA molecule with enzymatic activity. *Aminoacyl tRNA synthetase* - **Aminoacyl-tRNA synthetases** are a family of enzymes responsible for **attaching the correct amino acid to its corresponding tRNA molecule**, a critical step in ensuring the accuracy of protein synthesis. - These are **protein enzymes** and do not possess RNA-based catalytic activity.

Q: According to Chargaff's rules, which of the following base pairs shows equal numbers in double-stranded DNA?

G=C. ***G=C*** - Chargaff's rules state that in double-stranded DNA, the amount of **guanine (G)** equals the amount of **cytosine (C)**. - This is due to **complementary base pairing** where G always pairs with C via three hydrogen bonds. - Both G=C and A=T are valid answers according to Chargaff's rules. *A=T* - According to Chargaff's rules, **adenine (A)** equals **thymine (T)** in double-stranded DNA. - A pairs with T via two hydrogen bonds in complementary base pairing. - This is also a correct statement of Chargaff's rules (equally valid as G=C). *A=G* - **Adenine (A)** and **guanine (G)** are both purines but do not pair with each other. - Their amounts are **not necessarily equal** in double-stranded DNA. *A=C* - **Adenine (A)** is a purine and **cytosine (C)** is a pyrimidine, but they do not form complementary base pairs. - Their amounts are **not equal** in double-stranded DNA.

Q: The term "modified base" refers to:

A purine or pyrimidine that has been altered. ***A purine or pyrimidine that has been altered*** - A **modified base** refers to a purine (adenine, guanine) or pyrimidine (cytosine, thymine, uracil) that has undergone **post-transcriptional or post-replicative chemical alteration**. - These modifications can include **methylation**, deamination, or other structural changes, leading to changes in their properties and functions within nucleic acids. *A standard nucleotide consisting of a purine or pyrimidine attached to deoxyribose* - This describes a **standard deoxyribonucleotide** (the building block of DNA), which is composed of a nitrogenous base (purine or pyrimidine), a deoxyribose sugar, and one to three phosphate groups. - A **modified base** is distinct from a standard nucleotide because it involves an *alteration* to the base itself, not just its assembly into a nucleotide. *A purine or pyrimidine attached to a sugar by O-glycoside linkage* - In nucleic acids, **purine and pyrimidine bases** are attached to the C1' carbon of the sugar (deoxyribose or ribose) via an **N-glycosidic bond**, not an O-glycosidic linkage. - An **O-glycosidic linkage** is typically found in carbohydrates, connecting sugars to other molecules, but is not characteristic of the bond between a base and a sugar in nucleotides. *A nitrogen-containing base that is not a standard purine or pyrimidine* - While modified bases are indeed **nitrogen-containing bases** that often are not the *standard* A, T, C, G, or U, this definition is incomplete. - The key aspect of a **modified base** is that it *originated* from a standard purine or pyrimidine and was subsequently *altered*, differentiating it from completely novel nitrogenous bases.

Q: The Feulgen reaction is a cytochemical test used to detect which of the following?

Deoxyribonucleic acid (DNA). ***Deoxyribonucleic acid (DNA)*** - The **Feulgen reaction** is a specific histochemical technique used to detect and quantify **DNA** in cells. - It relies on the hydrolysis of purine bases from the DNA molecule, which exposes aldehyde groups that then react with **Schiff's reagent** to produce a magenta color. *Transfer RNA (t-RNA)* - tRNA is a type of **RNA** molecule that is involved in protein synthesis. - The Feulgen reaction is specific for **DNA** and does not detect **RNA** because RNA lacks the deoxyribose sugars that are critical for the reaction mechanism. *None of the options* - This option is incorrect because **DNA** is a correct answer, as the Feulgen reaction specifically targets and stains DNA. - The specificity of the Feulgen reaction for DNA makes it a widely used tool in cytogenetics and cell biology. *Messenger RNA (m-RNA)* - mRNA is a type of **RNA** that carries genetic information from DNA to ribosomes for protein synthesis. - Like other RNA molecules, **mRNA** does not react with the Feulgen stain due to the presence of **ribose sugar** instead of deoxyribose, which prevents the formation of aldehyde groups needed for the reaction.

Q: Which of the following pairs are purines that utilize the salvage pathway?

Adenine & Guanine. ***Correct: Adenine & Guanine*** - **Adenine** and **guanine** are the two major **purine bases** that are actively salvaged through dedicated salvage pathways to form their respective nucleotides - **Adenine** is salvaged by **APRT (adenine phosphoribosyltransferase)** to form AMP - **Guanine** is salvaged by **HGPRT (hypoxanthine-guanine phosphoribosyltransferase)** to form GMP - The salvage pathway conserves energy by recycling free bases rather than synthesizing nucleotides de novo - This is the **most complete and accurate pair** for the question *Incorrect: Hypoxanthine & Thymine* - **Thymine** is a **pyrimidine**, not a purine, making this pair incorrect - While hypoxanthine is a salvageable purine (via HGPRT to form IMP), the pair fails the "both purines" requirement *Incorrect: Xanthine & Guanine* - While **guanine** is correctly salvaged by HGPRT, **xanthine** is primarily a **degradation product** rather than a salvage substrate - Xanthine is an intermediate in purine catabolism that is oxidized to **uric acid** by xanthine oxidase - Xanthine is **not a significant substrate for salvage pathways** to form functional nucleotides *Incorrect: Hypoxanthine & Xanthine* - **Hypoxanthine** is indeed salvaged by HGPRT to form IMP and is an important salvage substrate - However, **xanthine** is primarily a **catabolic intermediate** converted to uric acid, not a major salvage pathway substrate - While both are purines, xanthine does not significantly participate in salvage to form nucleotides, making this pair incomplete for the question's requirement

Question 1

Which of the following is NOT present in DNA?

Accepted Answer

Uracil

Answer

Thymine

Answer

Adenine

Answer

Cytosine

Question 2

DNA and RNA both contain which pyrimidine base?

Accepted Answer

Cytosine

Answer

Uracil

Answer

Thymine

Answer

Guanine

Question 3

Which of the following statements about base excision repair is correct?

Accepted Answer

Uses the enzyme DNA glycosylase to remove damaged bases and create an apurinic site

Answer

Used for the repair of deaminated bases only

Answer

Removes approximately 1 nucleotide at a time

Answer

Repairs bulky helix-distorting lesions caused by UV radiation

Question 4

Which one of the following statements about chromatin is not true?

Accepted Answer

H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes

Answer

DNA winds approximately 1.75 times around the nucleosomes

Answer

Covalent modification of histones influence chromatin compaction

Answer

Non-histone proteins are part of mitotic chromosomes

Question 5

RNA with enzymatic activity is

Accepted Answer

Peptidyl transferase

Answer

Cytidine deaminase

Answer

Peptidase

Answer

Aminoacyl tRNA synthetase

Question 6

According to Chargaff's rules, which of the following base pairs shows equal numbers in double-stranded DNA?

Accepted Answer

G=C

Answer

A=C

Answer

A=G

Answer

A=T

Question 7

The term "modified base" refers to:

Accepted Answer

A purine or pyrimidine that has been altered

Answer

A purine or pyrimidine attached to a sugar by O-glycoside linkage

Answer

A standard nucleotide consisting of a purine or pyrimidine attached to deoxyribose

Answer

A nitrogen-containing base that is not a standard purine or pyrimidine

Question 8

The Feulgen reaction is a cytochemical test used to detect which of the following?

Accepted Answer

Deoxyribonucleic acid (DNA)

Answer

Transfer RNA (t-RNA)

Answer

None of the options

Answer

Messenger RNA (m-RNA)

Question 9

Which of the following pairs are purines that utilize the salvage pathway?

Accepted Answer

Adenine & Guanine

Answer

Hypoxanthine & Thymine

Answer

Xanthine & Guanine

Answer

Hypoxanthine & Xanthine

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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